1. Field of the Invention
The present invention relates to a ground coil for a magnetically levitated railway, and more particularly to a levitation and guidance ground coil fixed to a side wall of a track.
2. Description of the Related Art
This kind of art is disclosed in, for example, in U.S. Pat. No. 4,913,059 to the applicant of the present invention.
FIG. 1 illustrates the connection and arrangement of such conventional ground electric coils.
Referring to FIG. 1, four unit coils 1 to 4 for levitation and guidance are arranged on the same side wall along the direction of travel of a levitated object. A levitating force is produced by connecting these coils 1 to 4 at levitation null-flux connecting portions 5 so that induced voltages due to linkage magnetic fluxes between the upper and lower coils counterbalance with each other when the levitated object runs. Furthermore, null-flux connecting wires 6 and 7 are also disposed between the opposing unit coils, that is, the coils 1 and 3 and the coils 2 and 4 so that induced voltages produced by the linkage magnetic fluxes counterbalance with each other.
Thus, the levitation and guidance coils serve the purposes of levitation and guidance independent of a propulsion conductor coil 9 which needs electricity from an outside power supply 8.
As described above, in the production of a ground coil for a magnetically levitated railway when large electrical insulating strength is not necessary, integral molding using a SMC (Sheet Molding Compound) in consideration of mass productivity and mechanical strength is popular. More specifically, the molding is carried out in the following procedures:
(1) A plurality of base coils are prepared on each of which a conductive wire is coiled in a desired shape in a plurality of turns. At this time, although it is natural that insulation be conducted on the wire, a preprocess is also necessary in preparation of subsequent impregnation or quenching. This preprocess means an operation necessary to perform temporary forming previous to actual molding, which is carried out, for example, in the following methods:
(a) In the case of temporary forming by impregnation, coils are bound by a glass tape which is excellent in permeability relative to impregnant resin and is strong in the pulling direction.
(b) In the case of temporary forming by quenching, wires and coils are bound by an insulating tape coated with a semicuring resin.
(2) The base coils are located in two tiers and connected in series by welding so as to form unit coils. At this time, a preprocess is also necessary in preparation of the subsequent impregnation or quenching.
(3) Two unit coils are connected so that the winding directions of them are opposite to each other and formed into a pair of levitation coils. When conductors of the unit coils are welded, guidance terminals are concurrently drawn out from welding points and levitation and guidance coils are produced. A preprocess is sometimes needed in preparation of the temporary forming to maintain an exact positional relationship between the upper and lower coils.
(4) In order to prevent the positional relationships between the wires of the coils and the coils from being changed by high pressure applied in molding of the coils, the temporary forming is carried out by resin impregnation or quenching previous to the molding.
(5) Two pairs of levitation and guidance coils are arranged on a single plane in a preheated metal mold in constant positional relationship.
(6) Molding materials are uniformly arranged in the metal mold so as to wrap the coils. At this time, it is necessary to take steps to prevent the coils from being displaced from respective predetermined positions.
(7) An upper lid is set on the metal mold, the molding materials are melted by heat and pressure, and then, integral molding is carried out by a thermosetting reaction.
However, the above conventional method of producing a ground coil has the following disadvantages:
(1) Since it is premised that the molding be carried out at high temperature under high pressure, equipment for heating and pressurizing the metal mold is necessary. In particular, when a large molded product is obtained, the equipment is enlarged.
(2) The metal mold is required to have a sufficient mechanical strength to withstand high temperature and high pressure, thereby increasing design and production costs.
(3) Since the coils are likely to be deformed by pressure in the molding, temporary forming by impregnation or quenching is necessary for the coils previous to actual molding.
(4) The preprocess of molding, that is, the positioning of the coils and the uniform arrangement of sheet molding materials are troublesome.
(5) Since a plurality of coils are integrally molded, it is likely that the relative positional relationship between the coils will change or that the arrangement of the molding materials will be made uneven.
(6) Since heat and pressure are applied from the periphery of the molding material through the metal mold, curing reaction gradually appears from the outer portion of the coils and thus inner stress is likely to be left.
(7) The curing and molding process takes a relatively long time.
(8) Since the specific gravity of the SMC material is relatively large, the weight of a finished integral coil including a plurality of coils is heavy. This has a bad influence on production and mounting operations.
(9) The molding material itself is highly expensive, and therefore, is not suitable for mass production.